PCB faults - the signs...

In general what are the signs of a faulty PCB in a boiler? Is there anything obvious? I have been reading various posts on this site out of interest and some say the board is the cause of a fault, eg/ Hot water fault, ignition fault, etc and just wondered what the signs / tests were to prove this.

I know there are some vary knowledgable/intelligent boiler engineers on this site who I enjoy reading their replies to various boiler posts and I was hoping someone would kindly explain this to me?

There's no straight answer to that question, PCB's have many components carrying out different functions. I could illustrate it by asking what are the signs of a faulty car? A car has many components and any could fail. If a gasket has gone, you may get oil on your drive, if a fuse has blown the headlights don't work, it the handbrake cable snaps the car will roll away.

With a boiler PCB controlling the fan, pump, gas valve, thermocouple, thermostat, overheat etc etc., depending on what component fails determines what doesn't work. For example if the power supply circuitry fails nothing would work, maybe a relay doesn't operate so only the pump doesn't run. Usually most are interlocked so that if the fan doesn't run, the burner doesn't light. If the pump doesn't run the overheat trips.

Modern boilers have diagnostics built in that give a whole host of fault codes to highlight what has failed.

What I guess I'm trying to say is, what happens depends on the component that fails. It's normally a process of working through the operating sequence and finding where it goes wrong or stops.

Having said that some PCB's tend to have common faults that crop up fairly regularly so those who have experience with a particular boiler may spot a pattern.

I'm an electrical engineer BTW, not a "boiler engineer" but I am familiar with PCB's and their failings.

I do understand operating sequences, how to fault find coponents of a boiler to a certain degree, but was just wondering what would cause a PCB to be the failure component. I know sometimes they can have burn marks on them, smell, etc .... also some may have a resistor etc and these can fail. So if a PCB was faulty, then the firing sequence of a boiler would stop and hence the boiler not work.

For eg/ if the fan's running but the boiler not firing - depending on boiler type, it could be a faulty APS diaphragm or microswitch/aps tube/blocked orifice/ etc

Basically, why would a PCB fail ie have brn marks, etc and how would you know? Would the boiler just not allow any parts to operate, ie not even the pump to run/fan to run etc??

Modern PCBs are computer based and its digital techniques which would be needed to identify a logic fault.

Of course thats not needed in practice. All that is needed is to be able to identify when the PCB is not working properly.

A boiler PCB has a harded task because it operates at low power levels in the logic parts but still has to control pump, fan and gas valve which consume higher powers. All this on one PCB.

Some PCB faults are quite minor and dont show up in normal use. I had to change a faulty PCB which would not accept an instruction to operate a heat only boiler although it was fine on a combi.

I have this niggling feeling from what you say that you are just wanting help to go out fixing boilers for people and wanting to know how you can simply look at a PCB in the hope of seeing a fault. Thats rarely the case and it needs a test meter and a lot of knowledge of electronics to find a fault on a PCB.

Obviously I would never encourage anyone who is not gas registered to go trying to repair boilers. Quite apart from the obvious gas parts, most PCBs have to be set up to correctly control the gas valve and combustion parameters.

Yes I am trying to learn/understand boilers and to fault find. I'm not doing it though, just interested in it. Have been for a while. I've been reading books/watching dvds/reading boiler manuals in my spare time (I don't like watching corrie/eastenders of an evening)

Over the past 18 months or 2 years I've learned basic boiler operation/fault finding on components such as Fan/APS/Pump/Flow Switches/DIverter Valves/Thermistors/Thermostats/Ignition electrode/leads/FSD's/checking for correct Polarity/ etc but was just curious about how to fault find a faulty PCB. I know it is very hard and not easy at all!

I have used a multi meter on non boiler components such as motorised valves/pumps/cylinder and room thermostats/programmers etc but never inside a boiler.

I know that a faulty board can smell/have burn marks/cracks in it and any water on the board can cause obvious faults/condensation etc inside the boiler ie from a leaking aav, etc can cause a board fault.
Thanks for your advice though about PCBs.

Hi,
A relay could fail in a couple of ways, the switching contacts could become pitted over time due to switching a load like a pump or a fan.
The winding of the relay could fail which would mean that it wouldn't operate.
Some of the relays on boiler PCB have a transparent cover, quite often you can see signs of scorching through this, but just because you can see some discoloring doesn't mean that the relay isn't working fine.

A lot of PCB's will be changed for a new one when the problem will be nothing more than a dry joint that a quick dab with a soldering iron would fix.
That said as Agile pointed out, boiler PCB's these days are becoming more & more complicated so the days of getting the boiler up & running with nothing more than a wave of the soldering iron will soon be gone I think.

I do understand operating sequences, how to fault find coponents of a boiler to a certain degree, but was just wondering what would cause a PCB to be the failure component. I know sometimes they can have burn marks on them, smell, etc .... also some may have a resistor etc and these can fail. So if a PCB was faulty, then the firing sequence of a boiler would stop and hence the boiler not work.

For eg/ if the fan's running but the boiler not firing - depending on boiler type, it could be a faulty APS diaphragm or microswitch/aps tube/blocked orifice/ etc

why do you want to know so badly, get a dog or a hobbie

Basically, why would a PCB fail ie have brn marks, etc and how would you know? Would the boiler just not allow any parts to operate, ie not even the pump to run/fan to run etc??